It may be months, if ever, before Ottawa gets to the bottom of who was responsible for the gangland-style murder of Canadian Sikh activist Hardeep Singh Nijjar. Given that Prime Minister Justin Trudeau has said there is “credible” intelligence that India played a role in the slaying, it’s hard to imagine any thorough investigation taking place that doesn’t include the full co-operation of the government of Indian Prime Minister Narendra Modi. At this point, that seems preposterous. Relations between the two countries are as bad as they’ve been in recent memory. Mr. Modi has called the allegations against his country...